Climate and Geology Overwrite Land Use Effects on Soil Organic Nitrogen Cycling on a Continental Scale

Soil fertility and plant productivity are globallyconstrained by N availability. Proteins are the largest N reservoir in soils,and the cleavage of proteins into small peptides and amino acids has beenshown to be the rate-limiting step in the terrestrial N cycle. However, weare still lacking a profound understanding of the environmental controls ofthis process. Here we show that integrated effects of climate and soilgeochemistry drive protein cleavage across large scales. We measured grossprotein depolymerization rates in mineral and organic soils sampled across a4000 km long European transect covering a wide range of climates, geologiesand land uses. Based on structural equation models we identified that soilorganic N cycling was strongly controlled by substrate availability, e.g., bysoil protein content. Soil geochemistry was a secondary predictor, bycontrolling protein stabilization mechanisms and protein availability.Precipitation was identified as the main climatic control on proteindepolymerization, by affecting soil weathering and soil organic matteraccumulation. In contrast, land use was a poor predictor of proteindepolymerization. Our results highlight the need to consider geology andprecipitation effects on soil geochemistry when estimating and predictingsoil N cycling at large scales.